1. Field of the Invention
The present invention relates to a zoom lens system, and more particularly, to a rear-focus-type zoom lens system which is compact in size.
2. Description of the Related Art
Electronic cameras are becoming increasingly smaller and lighter. As a result, when a zoom lens system is utilized, the same must also be small in size and light in weight. To achieve this, the zoom lens system is made shorter, an effective diameter of a first lens nearest to a photographic object is decreased, and the overall structure is simplified.
The zoom lens system generally includes first, second, third, and fourth lens groups. The first lens group has a positive refractive power and is provided closest to a photographic object. The second lens group has a negative refractive power and moves along an optical axis for variable magnification. The third lens group has a negative refractive power and maintains a fixed image plane while the magnification is varied. The fourth lens group has a positive refractive power and is held at a fixed position closest to an image plane.
A lens system having a greater size and weight is required when focusing by moving a first lens group inwardly and outwardly. However, with increasing use of motors for focusing lens systems, inner and rear focus methods are becoming more commonplace. Using these focusing methods, the total length of the optical system is not varied while the lens groups are driven. Thus, it is possible to minimize the size of the motor.
When focusing using the above rear focus method, a fourth lens group, out of a total of four lens groups, moves along an optical axis.
In a lens system utilizing the rear focus method, an effective diameter of the first lens group is typically smaller than that of the zoom lens system in which the first lens group is moved. Using the rear focus method, it is easier to minimize the size of the entire lens system. Furthermore, because it is easy to take pictures extremely close to an object and because it is easier to move the relatively smaller and lighter lens groups, speedy focus control is possible.
However, large aberration variations occur when focusing with the above rear-focus-type zoom lens. In addition, it is difficult to design the lens system to be lightweight and compact while enabling high optical performance over an entire range of short and remote distances from an object. Furthermore, when a wide angle and rear-focus-type zoom lens is used, it is difficult to achieve a high optical performance over an entire variable magnification range.
Conventional rear-focus-type zoom lens systems have been proposed in the following patents:
(1) Japanese Laid-Open Patent Application No. Sho 63-44614, PA1 (2) Japanese Laid-Open Patent Application No. Hei 3-259209, PA1 (3) Japanese Laid-Open Patent Application No. Hei 8-5920, PA1 (4) Japanese Laid-Open Patent Application No. Hei 7-270684, and PA1 (5) Japanese Laid-Open Patent Application No. Hei 5-72474.
The lens systems described in Japanese Laid-Open Patent Application Nos. Sho 63-44614, Hei 3-259209, and Hei 8-5920 have first, second, third, and fourth lens groups. The first lens group has a positive refractive power and is positioned closest to a photographic object. The second and third lens groups have a negative refractive power. The fourth lens group has a positive refractive power. The first and the fourth lens groups are fixed, while the second and the third lens groups move for variable magnification.
Although the lens systems described in Japanese Laid-Open Patent Application Nos. Sho 63-44614 and Hei 3-259209 perform well optically, they require substantial drive power for varying magnification because the variable magnification lens groups have four or more lenses. Furthermore, because the distance between the second and third lens groups is small, it is difficult to manufacture these lens systems.
In Japanese Laid-Open Patent Applications Nos. Hei 7-270684, and Hei 5-72474, first and third lens groups are fixed, a second lens group performs a variable magnification, and a fourth lens group both focuses and compensates for image plane movement occurring during variable magnification.
However, the lens systems described in Japanese Laid-Open Patent Application No. Hei 7-270684 has a high level of residual aberration, making it difficult to achieve a high resolution level. Also, although the lens system described in Japanese Laid-Open Patent Application No. Hei 5-72474 has a relatively simple structure and performs well optically, it is difficult to manufacture because two lenses that are aspherical on both sides are used.